An antenna is a transducer designed to transmit and/or receive radio, television, microwave, telephone and radar signals, i.e. an antenna converts electrical currents of a particular frequency into electromagnetic waves and vice versa. Physically, an antenna is an arrangement of one or more electrical conductors that is arranged to generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or that can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals.
Portable wireless communication electronic devices, such as mobile phones, typically include an antenna that is connected to electrically conducting tracks or contacts on a printed circuit board by soldering or welding. Manufacturers of such electronic devices are under constant pressure to reduce the physical size, weight and cost of the devices and improve their electrical performance. This low cost requirement dictates that the electronic device and its antenna should be simple and inexpensive to manufacture and assemble.
A further challenge facing manufacturers is to provide electronic devices with a compact, high gain, multi-band antenna i.e. an antenna capable of simultaneously transmitting and/or receiving signals using different wireless communication standards, such as GPS, Rx diversity, W-LAN, Wi-Fi, Bluetooth and UWB, with a good front to back ratio.
The front to back ratio of an antenna (which is usually expressed in dB,) is defined as the gain of the antenna in a specified direction, usually that of maximum gain, compared to the gain in a direction 180° from the specified direction. The front to back ratio is normally defined as the ratio of radiation, away from the user's head compared with the ratio of radiation towards the user's head. An antenna which radiates as much towards the user's head as away from it (when a user is holding a call phone containing the antenna in talking position) is said to have a 0 dB front to back ratio. An antenna which radiates 1 dB more away from the user's head than towards the user's head is said to have a 1 dB front to back ratio. The higher this ratio the better the talking performance of a mobile phone containing the antenna.
It has been found that many conventional antennas, when placed at the bottom of a stick type mobile telephone over a ground plane, have a front to back ratio of approximately 0 dB. It is also well known that planar inverted F antennas (PIFAs) over a ground plane can be used to improve the front to back ratio. It is however also well known that PIFA antennas have narrow low-band bandwidth and that it is difficult to achieve good performance in the 850 and 900 MHz GSM bands if the antenna occupies small volume.